1. Field of the Invention
The present invention relates to color processing for estimating characteristics of illumination light.
2. Description of the Related Art
It becomes a common practice to input images from an image input device such as a digital camera or scanner to a personal computer (PC), to display the images on a display device such as a liquid crystal display (LCD), and to output the images using an output device such as a printer. Then, a color management technique which absorbs color reproduction differences between various devices and corrects colors for any purpose becomes important. That is, image data undergoes color conversion by the color management technique to attain color matching between, for example, an image to be displayed on a monitor and that to be output by a printer.
However, color appearance largely varies depending on viewing illuminations of images. Even when color matching is attained under a certain condition, color matching is disabled if a viewing illumination has changed. For example, fluorescent lamps, which are generally used in offices and home, are classified based on their color temperatures and color rendering property types, and color appearance changes depending on their types.
The color temperatures of the fluorescent lamps are classified into five indices, that is, “incandescent”, “warm white”, “white”, “neutral white”, and “daylight”. Also, the color rendering property types of the fluorescent lamps are classified into three types, that is, “broad band”, “three band”, and “normal”. The color temperatures have warm colors as they are lower, and have cool colors as they are higher. The index “incandescent” has a warmest color, and the indices “warm white”, “white”, “neutral white”, and “daylight” have colors closer to cool colors in the order named.
The color rendering property is an index indicating how much a color of an object illuminated with illumination light is closer to that when the object is illuminated with sunny daylight outdoors. That is, as the color rendering property is higher, a color under sunny daylight outdoors is reproduced more faithfully. A fluorescent lamp of a “broad band” type has the highest color rendering property, and is used in business places associated with colors, art museums, and the like. A fluorescent lamp of a “three band” type has the second highest color rendering property, and is popularly used in home, offices, and the like. A fluorescent lamp of a “normal” type has the lowest color rendering property, and is used in general offices, warehouses, and the like in which the color appearance does not mean much.
In this manner, when only fluorescent lamps are covered, there are many types of fluorescent lamps, and accurate color matching cannot be expected unless the characteristics of viewing illuminations are taken into consideration. For example, when an image, which matches a monitor color under a bluish white illumination such as a “daylight” fluorescent lamp, is output, if that image is viewed under a reddish illumination such as a “incandescent” fluorescent lamp, the entire image is reddish in color, and does not match a color of the image displayed on the monitor.
In order to solve such problem, characteristics of viewing illuminations have to be acquired, and color matching processing suited to the acquired characteristics has to be executed. Japanese Patent Laid-Open No. 2008-271303 (literature 1) describes an invention which executes color matching processing suited to viewing illuminations. According to literature 1, a chart on which a plurality of images, obtained by color-correcting a plurality of memory colors in correspondence with a plurality of light sources, are laid out is output. Then, the control prompts the user to select a most preferred image of the chart under a viewing illumination. That is, the color temperature index and color rendering property type of the viewing illumination are determined based on the image which is visually selected by the user.
However, determination of the color temperature index and color rendering property type of the viewing illumination in literature 1 depends on a visual organoleptic examination of the user, and an accurate determination result cannot always be obtained. Of course, when the viewing illumination is measured, an accurate determination result can be obtained. However, expensive measuring devices are required, and expert knowledge is also required to operate the measuring devices.